The rack and pinion type steering device for the vehicle is so designed as to transmit the rotation of a pinion to a rack that is meshed with the pinion to move a tie rod that is fitted to an end portion of the rack, and transmit the rotation to a steering unit that controls the direction of the tire wheels.
The rack and pinion type steering device of this type includes a rack guide that presses the back surface of a rack shaft in a meshing direction by the aid of an elastic body such as a spring so that the pinion and the rack are appropriately meshed with each other.
Also, the rack guide is of a slide type in which the rack shaft and the rack guide are brought in slide contact with each other, and of a rolling type in which the rack shaft is supported by a roller. The rolling type is so configured as to bear a pin that supports the roller by a pin insertion groove that is defined in a rack guide holder (refer to Japanese Laid Open Patent Publication No. 2004-034829).
FIG. 5 is a cross-sectional view for explaining an example of the configuration of a rack and pinion type steering device 100 having the above conventional rolling type rack guide, FIG. 6(a) is a partially cross-sectional view taken along a line A-A, FIG. 6(b) is a cross-sectional view taken along a line B-B of FIG. 5 from which a housing is omitted.
A rack and pinion type steering device 100 is so configured as to arrange a pinion shaft 104 and a rack shaft 105 in the interior of a housing 101. The pinion shaft 104 is rotationally supported by a ball bearing 102 and a needle bearing 103. The rack shaft 105 is so arranged as to be movable in the axial direction by the aid of a rack bush not shown. An end of the rack shaft 105 is coupled with a tie rod having a link unit that changes the direction of the tire wheels through a ball joint. A rack tooth 105a of the rack shaft 105 is meshed with a pinion tooth 104a of the pinion that is integrally formed with the above pinion shaft 104.
Further, a rack guide 106 is disposed at an opposite side of the pinion shaft 104 with respect to the rack shaft 105 in the interior of the housing 101. The rack guide 106 is so configured as to press the rack shaft 105 from the back surface to appropriately maintain a meshing state of the pinion tooth 104a with the rack tooth 105a. 
The rack guide 106 is made up of a rack guide holder 107 that is totally formed in a substantially cylindrical shape, a pin 108 that is arranged in a pin support hole 107a which is defined in an inner space of the rack guide holder 107 in a direction orthogonal to the axial direction of the rack shaft 105, and a roller 110 having a needle bearing 108 pressed into a center portion thereof and having an outer peripheral surface formed in a hand drum shape.
The roller 110 is installed on the pin 108 and rotationally disposed in the inner space of the rack guide holder 107. The outer peripheral surface of the hand drum shape of the roller 110 is brought in rolling contact with the back surface of the rack shaft 105 (a surface at an opposite side of the meshed surface) so as to press the rack shaft 105 toward the meshed surface.
The housing 101 is equipped with a rack guide portion 111 having a cylindrical aperture that guides the rack guide holder 107, and the outer peripheral surface of the rack guide holder 107 is fitted with the rack guide portion 111. Also, a screw is formed in the inner surface of the rack guide portion 111 on a lower side (on an opposite side of the rack shaft 105) of the rack guide portion 111 of the housing 101, so as to be meshed with an adjustment screw 112.
The adjustment screw 112 is formed of a cylindrical member having a bottom. The adjustment screw 112 is so configured as to be meshed with the rack guide portion 111, and press the rack guide holder 107 toward the rack shaft 105 through a disc spring 113 interposed between the adjustment screw 112 and the rack guide holder 107. The screwing amount of the adjustment screw 112 is so adjusted as to appropriately adjust the meshing state of the rack tooth 105a with the pinion tooth 104a. The rack guide holder 107 can be displaced by the amount of elastic deformation of the disc spring 113.
The conventional rack guide described with reference to FIGS. 5, 6(a), and 6(b) suffers from the problems described below. That is, FIG. 7 is a diagram for explaining the cross-sectional configuration of the outer peripheral surface of the hand drum shape of the conventional roller 110. In the conventional rack guide, as shown in FIG. 7, the cross-sectional configuration of the hand drum shaped outer peripheral surface of the roller 110 is made up of curved surfaces R1 and R2 (R1 can be equal to R2) consisting of two circular arcs having the radius of curvature larger than the radius of curvature RR of the outer peripheral surface of the rack shaft 105. Therefore, the hand drum shaped outer peripheral surface of the roller 110 and the outer peripheral surface of the rack shaft 105 are brought in point contact with each other at two points A and B.
The reason is that the radius of curvatures R1 and R2 of the outer peripheral surface of the roller 110 and the radius of curvature RR of the outer peripheral surface of the rack shaft 105 cannot be manufactured in the entirely identical radius of curvature because of the tolerance (permissible error) in the manufacture.
However, when the roller 110 and the rack shaft 105 are brought in point contact with each other as described above, because an area of the contact portion is very small, an estrangement force that occurs when the pinion and the rack are meshed with each other is transmitted to the rack guide holder 107. Then, the contact portion of the roller 110 with the rack shaft 105 becomes high surface pressure, and the contact portion is liable to be worn.
As the countermeasure against the wear of the contact portion, it is general to increase the hardness of the contact portion, and in the above structure, there is proposed that the rack shaft is made of high carbon steel, and the roller is made of high carbon chromium bearing steel, and the rack shaft and the roller are subjected to a heat treatment to increase the hardness.
However, even if the hardness of the roller and the rack shaft is increased, the contact portion is high surface pressure without any change, and the wear cannot be completely suppressed. Also, the roller is deformed in the heat treatment, and the fluctuation of the roller becomes large with respect to the rotation center. When the vibration exists in the roller, the amount of elastic deformation of the disc spring (refer to FIG. 5) changes by the fluctuation amount due to the phase (rotational angle position) of the roller. As a result, since the movable amount of the rack guide changes, the fluctuation of the roller must be prevented as much as possible. For that reason, after the roller has been subjected to the heat treatment, the outer surface of the roller is ground so as to eliminate the fluctuation. However, the costs increase because the grinding process is conducted.
As described above, even if the hardness of the roller and the rack shaft is increased, the wear of the contact portion cannot be completely eliminated although the costs increase. As a result, there occur disadvantages such as an increase in the amount of elastic deformation of the disc spring or an increase in the movable amount of the rack guide.
Also, in the rack and pinion type steering device, when the rack guide movable amount increases, gear rattle (rattle noise) occurs. For that reason, an increase in the excessive rack guide movable amount must be avoided.
An object of the present invention is to solve the above disadvantages, that is, to suppress the wear of the contact portion of the roller with the rack shaft, eliminate an increase in the excessive rack guide movable amount, and prevent the rattle noise from occurring.